Heretofore, as a working fluid such as a refrigerant for a refrigerator, a refrigerant for an air-conditioning apparatus, a working fluid for power generation system (such as exhaust heat recovery power generation), a working fluid for a latent heat transport apparatus (such as a heat pipe) or a secondary cooling fluid, a chlorofluorocarbon (CFC) such as chlorotrifluoromethane or dichlorodifluoromethane or a hydrochlorofluorocarbon (HCFC) such as chlorodifluoromethane has been used. However, influences of CFCs and HCFCs over the ozone layer in the stratosphere have been pointed out, and their use is regulated at present. In this specification, abbreviated names of halogenated hydrocarbon compounds are described in brackets after the compound names, and the abbreviated names are employed instead of the compound names as the case requires.
Under the above conditions, as a working fluid for heat cycle, a hydrofluorocarbon (HFC) which has less influence over the ozone layer, such as difluoromethane (HFC-32), tetrafluoroethane or pentafluoroethane (HFC-125) has been used. For example, R410A (a pseudoazeotropic mixture refrigerant of HFC-32 and HFC-125 in a mass ratio of 1:1) is a refrigerant which has been widely used. However, it is pointed out that HFCs may cause global warming. Accordingly, development of a working fluid for heat cycle which has less influence over the ozone layer and has a low global warming potential and which can replace R410A, is an urgent need.
For example, 1,1,1,2-tetrafluoroethane (HFC-134a) used as a refrigerant for an automobile air-conditioning system has a global warming potential so high as 1,430. Further, in an automobile air-conditioning system, the refrigerant is highly likely to leak out to the air e.g. from a connection hose or a bearing.
As a refrigerant which replaces HFC-134a, carbon dioxide and 1,1-difluoroethane (HFC-152a) having a global warming potential of 124 which is low as compared with HFC-134a, have been studied.
However, with carbon dioxide, the equipment pressure tends to be extremely high as compared with HFC-134a, and accordingly there are many problems to be solved in application to all the automobiles. HFC-152a has a combustion range, and has a problem for securing the safety.
As a working fluid for heat cycle, a hydrofluoroolefin (HFO) having a carbon-carbon double bond has been used, which has less influence over the ozone layer and has less influence over global warming, since the carbon-carbon double bond is likely to be decomposed by OH radicals in the air.
As a HFO to be used for the working fluid for heat cycle, for example, Patent Document 1 proposes 3,3,3-trifluoropropene (HFO-1243zf), 1,3,3,3-tetrafluoropropene (HFO-1234ze), 2-fluoropropene (HFO-1261yf), 2,3,3,3-tetrafluoropropene (HFO-1234yf) and 1,1,2-trifluoropropene (HFO-1243yc).
Further, as a HFO used as a working fluid for heat cycle, Patent Document 2 discloses 1,2,3,3,3-pentafluoropropene (HFO-1225ye), trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)), cis-1,3,3,3-tetrafluoropropene (HFO-1234ze(Z)) or HFO-1234yf.
As a working fluid for heat cycle having excellent refrigerant performance, a composition comprising trifluoroethylene (HFO-1123) (for example, Patent Document 3) has been known. Patent Document 3 also discloses an attempt to obtain a working fluid comprising HFO-1123 and various HFCs or HFOs in combination for the purpose of increasing the flame retardancy, cycle performance, etc. of the working fluid.
However, HFOs disclosed in Patent Documents 1 and 2 are insufficient in the cycle performance (capacity) and among such HFOs, one having a low proportion of fluorine atoms has combustibility. Further, HFOs disclosed in Patent Document 2 are also insufficient in the cycle performance (capacity).
Further, Patent Document 3 failed to disclose or suggest to obtain a working fluid comprising HFO-1123 and a HFC or another HFO in combination with a view to obtaining a working fluid for heat cycle which is practically useful comprehensively considering the balance of the capacity, the efficiency, the safety and the discharge temperature. For example, HFO-1123 may undergo self-decomposition reaction under high temperature and high pressure conditions, and when a composition containing HFO-1123 is to be practically used, durability of a working fluid for heat cycle containing HFO-1123 should be improved. Further, if a compressor discharge gas temperature (hereinafter sometimes referred to as a discharge temperature) when a working fluid for heat cycle is applied to a refrigerating cycle is high, the heat resistance of a material constituting the compressor, a refrigerant oil usually contained in a composition for a heat cycle system in addition to the working fluid, an organic compound and the like may be impaired.
Accordingly, a working fluid for heat cycle which has a small temperature glide and a sufficiently high cycle performance (capacity) and has less influence over global warming, and which further has a sufficiently low discharge temperature and higher durability, has been desired.